1. Field of Invention
The present invention relates to the growth of III-nitride light emitting devices.
2. Description of Related Art
Semiconductor light-emitting diodes (LEDs) are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness LEDs capable of operation across the visible spectrum include Group III-V compound semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials. Such devices typically have a light emitting or active region sandwiched between a p-doped region and an n-doped region. The active region may be a single light emitting layer or multiple quantum well layers separated by barrier layers. Often III-nitride devices are epitaxially grown on sapphire, silicon carbide, or III-nitride substrates by metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. Devices grown on a conductive substrate often have the contacts formed on opposite sides of the substrate. Alternatively, the device may be etched to expose portions of both the n- and p-type regions on the same side of the substrate in devices grown on poorly conducting substrates, or for optical or electrical reasons. The contacts are then formed on the exposed regions. If the contacts are reflective and light is extracted from the side of the device opposite the contacts, the device is referred to as a flip chip. III-nitride LEDs structures are often grown on sapphire substrates due to sapphire's high temperature stability and relative ease of production.
The use of a sapphire substrate may lead to poor extraction efficiency due to the large difference in index of refraction at the interface between the semiconductor layers and the substrate. When light is incident on an interface between two materials, the difference in index of refraction determines how much light is reflected at that interface, and how much light is transmitted through it. The larger the difference in index of refraction, the more light is reflected. The refractive index of sapphire (1.8) is low compared to the refractive index of the III-nitride device layers (2.4) grown on the sapphire. Thus, a large portion of the light generated in the III-nitride device layers is reflected when it reaches the interface between the semiconductor layers and a sapphire substrate. The reflected light is waveguided and makes many passes through the device before it is extracted. These many passes result in significant attenuation of the light due to optical losses at contacts, free carrier absorption, and interband absorption within any of the III-nitride device layers.